government's been dissolved by the axis powers
people begin to lose faith in their government's ideology
some states, believing that the superior fascist rule (as shown with germany's victory over the usa) is the key to becoming a world power, adopt it
other states, believing that communism, which has stood tall against the fascist pillar as an equal, see the benefits of such a communal approach
or the others who believe that there was nothing wrong with democracy, or simply that the form of democracy the original government was providing was not sufficient and believe that they can do better, adopt it
when the original country has been completely destroyed, it's pretty easy to suspend your disbelief and play along that some states would be arrogant enough to become fascist or communist